1. Field of the Invention
This invention relates to electric furnaces and to methods of their operation for heating thermoplastic material, particularly molten glass having a relatively high electrical resistivity at molten temperatures.
In the discussion of the invention which follows, particular reference will be made to glass as the exemplary thermoplastic material to be processed in the furnace. However, it is to be understood that other materials having high electrical resistivity at molten state processing temperatures can be processed in apparatus of the type proposed by methods as suggested herein with similar advantages to those attendant the glass processing to be discussed.
2. Description of the Prior Art
The manufacture of glass in large quantities has generally been accomplished by melting and refining the glass constituents in relatively large chambers which have been heated by fossil fuel firing. Environmental considerations and the increasing scarcity of fuel has recently imposed particular emphasis on electrical heating as a means of melting and refining glass. Such heating, particularly where a continuous blanket of unmelted batch is floated on the surface of the molten glass over all or a substantial portion of the free surface of materials constrained by the furnace, results in a savings in heat by virtue of the batch blanket thermal insulating properties, a reduction in the temperature of the gas released to the atmosphere and a reduction in the emission of gaseous constituents from the glass melting and refining process.
In the case of those glasses of relatively low electrical resistivity at their melting and working temperatures such as alkali borosilicate and soda-lime silicate glasses, Joule effect heating wherein current is passed through the molten glass to cause that glass to function as a heating resistance has been utilized effectively. However, glass and other thermoplastic materials having relatively high electrical resistivities at those working temperatures at which they are molten have presented problems which have not been satisfactorily resolved for large scale commercial production.
Particular difficulty has been experienced in attempts to apply Joule effect heating to the production in commercial quantities of the glass composition commonly known as E Glass typically comprising:
SiO.sub.2 54% Al.sub.2 O.sub.3 14% CaO 17.5% MgO 4.5% B.sub.2 O.sub.3 10%
Such glass requires a high temperature for melting and refining, has a relatively high electrical resistivity, and has a relatively steep negative temperature coefficient of electrical resistance.
Refractory materials employed in the receptacles for molten glass in the melting, refining and working of that glass have a useful life which is primarily determined by the rate of loss of the refractory material. The investment involved in construction and rebuilding of such receptacles dictates a requirement for a life of several years for a continuous furnace wherein, in normal operation, batch material is fed and glass is withdrawn on a continuous basis.
A preferred wall material for molten glass receptacles is chromic oxide. Typically E glass is processed in the range from 2400.degree. to about 2750.degree. F. At these temperatures, chromic oxide refractories function satisfactorily over the required furnace life when subjected to a fossil fuel firing. That is, they are mechanically eroded by the glass constituents and are thermally and chemically corroded at rates which are economically tolerable. However, when attempts have been made to heat E glass by Joule effect heating, chromic oxide refractories have been found to deteriorate an excessive rate, unless special precautions are taken.
One means of protecting chromic oxide refractory is disclosed in George R. Machlan U.S. Pat. No. 3,806,621 of Apr. 23, 1974, for "Electric Furnace", wherein at least one side wall and usually all side walls of the melting chamber are formed from a low solubility chromic oxide refractory, and the bottom is formed from a refractory having a high electrical resistance at the temperature of the molten glass, and wherein a plurality of electrodes are positioned in the chamber to at least partially surround one or more cooperating, interconnected electrodes, to limit the maximum potential difference or electrical stress in the side walls as might be imposed by the voltage applied between the electrodes to accomplish the Joule effect heating. The positioning of the interconnected electrodes between the wall and the cooperating electrode or plurality of electrodes provides a fence or guard ring with respect to the potential difference or electrical stresses in the side wall. In one arrangement, the fence or guard electrodes are grounded and the electrode or electrodes which they surround are connected to the hot side of the power source.
A typical refractory offering relatively high electrical resistivity at glass melting temperatures is dense zircon. This material is employed as the furnace bottom in the Machlan patent. While the dense zircon refractory has a substantially higher electrical resistivity than E glass at the temperatures of molten E glass it is subject to rapid deterioration by erosion and corrosion when employed in the side walls of the glass tank.
Heretofore, a number of patent disclosures have presented techniques of limiting the amount of electrical heating which occurs in the molten glass in the vicinity of the side walls of a glass melting receptacle. Cornelius U.S. Pat. Nos. 2,089,689 and 2,089,690 of Aug. 10, 1937 each entitled "Electric Furnace" disclose furnace constructions wherein electrodes pass through the side walls of a glass melting receptacle, or at least have electrically conductive elements contacting those side walls and extending between the source of electrical power and the electrodes in engagement with the glass in the receptacle. Additional patent disclosures which involve the special orientation of electrodes within a glass melting furnace to avoid localized heating of the molten glass in the vicinity of the walls include Romazzotti U.S. Pat. No. 2,267,537 of Dec. 3, 1941 for "Electric Furnace", Borel et al. U.S. Pat. No. 2,552,395 of May 8, 1951 for "Electric Glass Furnace", Lambert U.S. Pat. No. 2,636,913 of Apr. 28, 1953 for "Method and Apparatus for the Manufacture of Glass by Electric Heating" and Penberthy U.S. Pat. No. 3,409,725 of Nov. 5, 1968 for "Furnace Electrode Assembly". While these prior art patents may have been satisfactory with respect to the glass compositions of relatively low electrical resistivity when in the molten state, when compositions of glass having a resistivity in the molten state greater than the resistivity of the containing refractory are processed in their arrangement, an intolerably high rate of refractory deterioration would be experienced, with Joule effect heating. For example, in the case of the Cornelius Patents, the low resistivity side walls would conduct a preponderance of the current resulting from the voltage applied across the electrodes, so that only a minor portion of the current would flow in the glass to produce Joule effect heating. It has been found that current flow in the refractory is a principal cause of refractory deterioration.
Erosion and corrosion are particularly troublesome in interfacial regions. In the case of electrodes for Joule effect heating of glass, the electrodes are attacked intensively at the air-glass, air-batch, and batch-glass interfaces such that electrode life is greatly reduced unless the electrodes are kept free of such interfaces and any oxidizing environments when at elevated temperatures. As disclosed in the aforenoted Machlan patent, the electrodes have been inserted through the furnace bottom to a level below the molten glass upper surface. This was not the case in the Cornelius disclosures and in the other cited patent disclosures, while the electrodes entered the melt below the melt upper surface, the walls through which they passed were not especially chosen to accommodate glass requiring high processing temperatures and having sidewalls of a lower electrical resistivity than the molten glass.